1. Field of the invention
The present invention relates to a light weight calcium silicate article having high mechanical strength. More particularly, the invention relates to a light-weight calcium silicate article having high mechanical strength, high heat resistance, and high dimensional stability which can be easily processed like wood.
2. Description of prior art
Calcium silicate-is light-weight and heat-stable material. Therefore, various calcium silicate articles utilizing such favorable properties are sold on market. Examples of such articles include a heat-insulation board utilizing the light weight property and a refractory calcium silicate board utilizing the heat-stable properties. Great amounts of both articles are produced in industry. The heat-insulation board has a bulk density of lower than 0.3 g/cm.sup.3, while the refractory board has a bulk density of higher than 0.7 g/cm.sup.3. Therefore, a calcium silicate article having a bulk density in the range of 0.3 to 0.7 g/cm.sup.3 is hardly available on the market.
The bulk density of 0.3 to 0.7 g/cm.sup.3 is almost the same as that of natural wood. Therefore, it has been proposed to produce a synthetic wood using calcium silicate, as stated below:
(1) Japanese Patent Provisional Publication No. 54(1979)-160428 describes a calcium silicate article which is prepared by molding an aqueous mixture of 100 weight parts of calcium silicate hydrate, 10-150 weight parts of hydraulic gypsum, 5-80 weight parts of a polymer emulsion, a flocculant for the polymer emulsion, water and a reinforcing fiber, and then drying the molded mixture; PA1 (2) Japanese Patent Provisional Publication No. 60(1985)-246251 describes a calcium silicate article which is prepared by molding an aqueous mixture of 100 weight parts of calcium silicate hydrate, 5-80 weight parts (as solid content) of carboxyl-containing styrenebutadiene copolymer latex, a cationic polymer flocculant, and water, and then drying the molded mixture; PA1 (3) Japanese Patent Provisional Publication No. 61(1985)-17462 describes a calcium silicate article which is prepared by molding an aqueous mixture of 100 weight parts (as solid content) of calcium silicate hydrate, 5-50 weight parts (as solid content) of a polymer emulsion prepared by polymerization of a hydrophobic ethylenic unsaturated monomer in the absence of an emulsifying agent, and 0.05-15 weight parts of a flocculant, and then drying the molded mixture; and PA1 (4) Japanese Patent Application No. 60(1985)-261811 filed in the name of the assignee of the present application describes a calcium silicate comprising 100 weight parts of calcium silicate hydrate, 1-30 weight parts of a reinforcing fiber, 1-40 weight parts of latex, and 0.1-5 weight parts of a dispersant. PA1 (1) an expansive cement selected from the group consisting of an expansive cement containing anhydrous calcium aluminosulfate (4CaO.3Al.sub.2 O.sub.3.SO.sub.3 O) which is named K-type, an expansive cement containing calcium aluminate cement and calcium sulfate which is named M-type and an expansive cement containing tricalcium aluminate and calcium sulfate which is named S-type. The names of K-type, M-type and S-type are defined in ACI (American Cement Institute) Classification and also in ASTM C-845; PA1 (2) calcium sulfoaluminate clinker, a mixture of calcium sulfoaluminate clinker and gypsum, or a mixture of aluminate clinker and gypsum; PA1 (3) a mixture of a) blast furnace slag or aluminum hydroxide-containing material, b) gypsum, and c) quick lime or slaked lime; and PA1 (4) a mixture of alumina cement and gypsum.
The heretofore proposed calcium silicate articles seem to have some drawbacks. Examples of the drawbacks are given below.
The calcium silicate article of (1) above does not have satisfactory mechanical strength. For instance, its specification teaches that bending strength of the article having bulk density. Of 0.5 g/cm.sup.3 is approx. 9.8 MPa (approx. 100 kgf/cm.sup.2) at the highest. Further, this article does not have satisfactory water resistance, because it contains a great amount of hydraulic gypsum. The incorporation of hydraulic gypsum in such a great amount further brings about other disadvantageous features such as safety, heat resistance, and dimensional B stability. In more detail, a calcium silicate article containing a great amount of gypsum is apt to produce toxic sulfur dioxide by decomposition of gypsum when kept at high temperatures such as 800.degree.-1,000.degree. C., or is apt to shrink in its dimension by releasing the hydrate water when kept at 100.degree.-500.degree. C.
The calcium silicate article of (2) above has an object to solve the problems attached to the calcium silicate article of (1) above such as unsatisfactory water resistance, safety, heat resistance and dimensional stability. However, almost no improvement is seen with respect to mechanical strength of the resulting article. For instance, the working examples given in the specification teach that bending strength of the article having bulk density of 0.5 g/cm.sup.3 is a value in the range of 40 to 110 kgf/cm.sup.2 (i.e., 3.9 to 10.8 MPa) at the highest. Further, the use of a great amount of such latex as styrene-butadiene copolymer latex (polymer emulsion) is not advantageous from the viewpoint of heat resistance, because the styrene-butadiene copolymer emulsion has high combustion enthalpy and the styrene content sometimes gives black smoke or malodor.
The calcium silicate article of (3) above has an object to solve the problems attached to the calcium silicate articles with respect to water resistance and mechanical strength, and has succeeded in improving the mechanical strength of a calcium silicate article to such extent that bending strength of the article having bulk density of 0.44-0.48 g/cm.sup.3 is 12.4-14.7 MPa (130-150 kgf/cm.sup.3). However, the resulting article is not satisfactory in heat resistance because a polymer emulsion of a great amount such as 30 weight parts (as solid content) is incorporated into the article.
The calcium silicate article of (4) above has an object to solve the problems attached to the calcium silicate articles of (1) and (2) above and has succeeded in improving the mechanical strength of a calcium silicate article to such extent that bending strength of the article having bulk density of 0.5 g/cm.sup.3 is approx. 11.8 MPa (approx. 120 kgf/cm.sup.3). However, the mechanical strength is preferably improved furthermore.